(a) Field of the Invention
The present invention relates to a display device and a driving method thereof. More particularly, the present invention relates to a display device having a reduced degradation of its driving transistors, and a driving method thereof.
(b) Description of Related Art
Recently, flat panel displays have been actively studied as substitutes for conventional cathode ray tubes (“CRT”). In particular, organic light emitting diode (“OLED”) displays are focused on as next-generation flat panel displays because of their good luminance characteristics and viewing angle characteristics.
Generally, an active matrix flat panel display includes a plurality of pixels arranged in a matrix and displays images by controlling the luminance of the pixels based on given luminance information. An OLED display is a self-emissive display device that displays images by electrically exciting light emitting organic material. An OLED display has low power consumption, wide viewing angle, and fast response time, thereby being advantageous for displaying motion images.
A pixel of an OLED display includes an OLED and a driving thin film transistor (“TFT”). The TFT includes polysilicon or amorphous silicon. A polysilicon TFT has several advantages, but it also has disadvantages such as the complexity of manufacturing polysilicon thin films, thereby increasing the manufacturing cost. In addition, it is difficult to make a large OLED display employing polysilicon TFTs.
On the contrary, an amorphous silicon TFT is easily applicable to a large OLED display and can be manufactured by a lesser number of process steps than the polysilicon TFT.
However, the threshold voltage of the amorphous silicon TFT shifts as time goes by due to a long-time application of a unidirectional voltage to a gate of the TFT such that the current flowing in the OLED under a given voltage is non-uniform thus degrading image quality and shortening the lifetime of the OLED.
Accordingly, a reverse bias voltage is suggested to be applied to the driving transistor for compensating the shift of the threshold voltage. However, the application of the reverse bias voltage may be insufficient for reducing the degradation of characteristics of the driving transistor such as the threshold voltage shift.